The present invention relates to a platinum alloy catalyst comprising a ternary platinum/rhodium/iron alloy on an electrically conductive carbon support. The ternary alloy is present in the form of finely divided alloy particles on the support material.
Platinum and in particular alloyed platinum catalysts on electrically conductive carbon supports are used as electrocatalysts for anodes and/or cathodes in fuel cells, preferably in phosphoric acid fuel cells (phosphoric acid fuel cell, PAFC) and polymer-electrolyte membrane cells (polymer electrolyte membrane fuel cell, PEMFC). Oxygen or air are used as the oxidizing agent on the cathode side. Hydrogen, hydrocarbons such as, for example, methane, higher alkanes or oxygen-containing hydrocarbons such as, for example, alcohols, or their reformates are used as fuels on the anode side. The conventional platinum loading is in the range 5 to 80, preferably in the range 10 to 50 wt. %, with respect to the total weight of catalyst. Carbon black, graphitized carbon black, graphite, carbides and their physical mixtures are used as electrically conductive carbon supports depending on the electrode face.
It is known that the electrical power produced by a phosphoric acid fuel cell depends substantially on the activity of the cathode catalyst. Therefore cathode catalysts are required which have a high current density for a given voltage and a low voltage drop during the lifetime of the catalyst. This leads to lower costs per unit of current produced.
A number of different platinum alloy catalysts have proven to be suitable catalyst systems with good power data, these mainly containing cobalt and chromium as other constituents.
Various processes for preparing these types of alloy catalysts are described in the patent literature. The individual processes differ substantially by the precursors used for the alloy components and by the type and manner of deposition of the alloy components on the conductive carbon support. The type and manner of deposition is a critical factor since the degree of dispersion of the subsequent alloy particles on the support, and thus the catalytically active metal surface area available for the electrochemical process, is largely determined by this process step.
After deposition on the support the alloy components may be reduced in a wet chemical process or in a gas phase reduction process.
All known processes complete preparation of the catalyst with a thermal treatment between 800 and 1000.degree. C. in an inert or reducing atmosphere. Reduction of the alloy components is completed during this thermal treatment.
The alloy catalysts, in comparison to pure platinum, are characterized in particular by improved ageing stability under the strongly corrosive conditions which exist during operation of a phosphoric acid fuel cell (operating temperature 170.degree. C. to 220.degree. C., 100% strength phosphoric acid as the electrolyte).
EP 0 665 985 B1 discloses the production of a ternary Pt/Rh/Fe catalyst which is intended to be characterised by an improved long-term stability as compared with the Pt/Co/Cr catalyst disclosed in U.S. Pat. No. 4,447,506 and in U.S. Pat. No. 4,677,092. EP 0 665 985 B1 is relied on and incorporated herein by references.
The Pt/Rh/Fe catalyst is produced in a multistage process, wherein in a first stage a supported, bimetallic platinum/rhodium catalyst is first prepared on a graphitized carbon black and this is impregnated with iron in a further step. An ordered Pt/Rh/Fe alloy catalyst with a cubic face-centred crystal structure is obtained by calcining the catalyst at temperatures between 800 and 1000.degree. C. and post-annealing at about 600.degree. C.
The process described for preparing the platinum alloy catalyst has a number of disadvantages.
Deposition of the alloy components on the support in EP 665 985 B1 is performed in several stages. This process is time-consuming and also sometimes requires a large reaction volume. The catalyst has a relatively high chlorine content in the individual manufacturing stages and this can have a negative effect on the activity, and in particular on the long-term stability, of the final catalyst. Therefore the chlorine is removed in subsequent washing processes which are sometimes associated with subsequent hydrolysis steps. There is no data given with respect to the diameters of alloy particles which can be produced.
To produce a ternary Pt/Co/Cr catalyst, U.S. Pat. No. 4,447,506 starts from a commercial platinum catalyst on graphitized carbon black, onto which cobalt and chromium are deposited by sequential impregnation. For this purpose, the platinum catalyst is dispersed in water with the aid of ultrasound for a period of 15 minutes. The pH of the suspension is adjusted to 8 with dilute ammonia solution. Then an ammonium chromate solution is added to the suspension and the pH is adjusted to 5.5 by adding dilute hydrochloric acid in order to deposit the chromium onto the platinum. After deposition of the chromium on to the platinum, a solution of cobalt nitrate is added to the suspension. The suspension is stirred thoroughly during addition of the solutions. After filtering off and drying the catalyst, it is calcined at 900.degree. C. for a period of 1 hour under a stream of nitrogen. Catalysts which have been prepared using this process have a non-ordered crystal structure.
According to U.S. Pat. No. 4,677,092 an ordered ternary alloy catalyst is obtained if, in addition to the process steps in accordance with U.S. Pat. No. 4,447,506, the platinum catalyst or the platinum alloy catalyst is cooled in an inert atmosphere, after calcination, at a rate which enables the production of an ordered crystal structure.
U.S. Pat. No. 5,013,618 describes the preparation of a ternary catalyst which contains a third metal in addition to platinum and iridium. To prepare this, carbon black, as a support material, is suspended in water with the addition of sodium bicarbonate. The suspension is boiled for 30 minutes with continuous stirring. Then a separately prepared solution of iridium chloride and hexachloroplatinic acid is added dropwise to the suspension. The resulting suspension is boiled for a further 15 minutes before formaldehyde is added to the suspension to reduce and precipitate platinum and iridium. After filtration the catalyst is washed with a solution of ammonium bicarbonate and dried at about 90.degree. C. The binary catalyst obtained in this way is then suspended in aqueous solution again and a water-soluble compound of the desired third metal is added thereto. The pH of the solution is adjusted to 5.5 with ammonia. The solution is evaporated to dryness and the solids remaining are calcined for 1 hour under nitrogen at 930.degree. C. in order to convert the deposited metals into the desired alloy. Then the temperature is lowered to 600.degree. C. and held at this value for a further hour before the catalyst is cooled to room temperature.
GB-OS 2,242,203 and EP 0450849 A2 describe the sequential preparation of platinum alloy catalysts in a one pot process. In the context of this invention, a one pot process is understood to be a procedure in which all intermediate stages of the catalyst are kept in liquid phase without any separation, drying or an optional calcining step during preparation. The process starts from hexachloroplatinic acid which is added to a basic suspension of a conductive carbon support. Sodium carbonate is used to make the support suspension basic and this also acts as a buffer to stabilize the pH of the solution. The alloy components cobalt and chromium are added to the suspension of non-reduced Pt/C catalyst as chlorides or nitrates. Alloy-formation is completed conventionally by thermal treatment under an inert gas atmosphere. A wet chemical reduction with hydrazine, formaldehyde or formic acid may be performed first or a gas phase reduction with hydrogen is an option.
DE 44 26 973 C1 describes another process for preparing a Pt/Co/Cr catalyst. The process involves making up an aqueous suspension of the carbon support, mixing this suspension with an aqueous solution of precursors of the alloy components, precipitating the alloy components in the form of their hydroxides by adding a base, reducing with a reducing agent, washing and drying the catalyst precursor obtained in this way and calcining at temperatures above 800.degree. C. to form the alloy. The essential point of the process is that the alloy components are nitrates which are all precipitated onto the carbon support simultaneously. The suspension of carbon support is heated to 80 to 90.degree. C. before adding the alloy components and is held at a constant temperature between 70 and 80.degree. C. after adding the alloy components. To complete reaction and also to deposit the alloy components, the pH of the suspension is raised to 8 to 9 by adding sodium hydroxide solution as a base. The use of alloy components in the form of nitrates is an essential factor in this process, in particular the use of platinum (IV) nitrate instead of the otherwise conventional hexachloroplatinic acid. Platinum(Iv) nitrate is characterized, in comparison to hexachloroplatinic acid, in that it can be hydrolysed very readily. The mutual precipitation of all three alloy components onto the carbon support is enabled by this property of the platinum (IV) nitrate.
An object of the present invention is to improve Pt/Rh/Fe alloy catalysts and to make them more active and more stable than traditional catalysts.
A further object of the present invention is to enable preparing this catalyst in a short time and therefore reduce production costs as compared with known processes.